MAC= line not specific enough… again

I think you missed this one @rickychilcott ;)

See: https://github.com/jnraine/munkiserver/pull/187 &
https://code.google.com/p/munki/issues/detail?id=197
All credit to @jps3 (Jason Stanford)
Merge pull request #191 from Ern-st/master

 MAC= line not specific enough… again